1. Field of the Invention
The present invention relates to a liquid crystal display (LCD) device, and more particularly, a backlight unit of an LCD device for improving light efficiency and color realization ratio.
2. Discussion of the Related Art
A Cathode Ray Tube (CRT), one of flat display devices, has been widely used for monitors of a television, a measuring machine and an information terminal. However, the CRT has limitations to miniaturization and lightness in weight due to a size and a weight in itself. Accordingly, display devices such as a liquid crystal display (LCD) device using an electro-optics effect, a plasma display panel (PDP) using a gas discharge and an Electroluminescence display (ELD) device using an electro-luminescence effect have been actively studied, which can substitute for the CRT.
Among the display devices, the LCD device is most actively studied, so that the LCD device having low power consumption, thin profile and lightness in weight is highly developed for being applied to monitors for a desktop computer and a large sized display device as well as for a laptop computer. Accordingly, demands for the LCD devices continuously increase.
Herein, the LCD device includes an LCD panel for displaying a picture image, and a driving part for applying a driving signal to the LCD panel. The LCD panel has first and second glass substrates bonded to each other at a predetermined interval, and a liquid crystal layer injected between the first and second glass substrates.
On the first glass substrate (TFT array substrate), there are a plurality of gate lines arranged in a first direction at fixed intervals, a plurality of data lines arranged in a second direction for being in perpendicular to the gate lines at fixed intervals, a plurality of pixel electrodes in respective pixel regions defined by the gate lines and the data lines in a matrix type, and a plurality of thin film transistors (TFTs) switchable in response to signals on the gate lines for transmission of signals on the data line to the pixel electrodes.
The second glass substrate (color filter substrate) has a black matrix layer for shielding light from areas excluding the pixel regions, a color filter layer (R, G, B) for displaying colors, and a common electrode for implementing a picture image.
The foregoing first and second glass substrates have a predetermined gap by spacers, and the first and second glass substrates are bonded by a sealant having a liquid crystal injection inlet. Then, liquid crystal is injected through the liquid crystal injection inlet.
Meanwhile, the LCD device controls transmittance of ambient light to display the picture image. In this respect, the LCD device requires an additional light source such as a backlight. The backlight is classified into a direct-type method and an edge-type method according to a position of a lamp unit.
The LCD device uses the light source such as an Electro Luminescence (EL), a Light Emitting Diode (LED), a Cold Cathode Fluorescent Lamp (CCFL) or a Hot Cathode Fluorescent Lamp (HCFL). Especially, the CCFL having long lifetime, low power consumption and thin profile is used as the light source for a large sized color TFT LCD device.
In case of the CCFL method, a fluorescent discharge tube is used for using a penning effect, which is formed by injecting a hydrargyrum gas containing Argon Ar and Neon Ne at a low temperature. Also, electrodes are formed at both ends of the fluorescent discharge tube, and the cathode is formed in a plate-shape. When a voltage is applied thereto, electric charges inside the fluorescent discharge tube collide against the plate-shaped cathode like a sputtering state, thereby generating secondary electrons. Thus, circumferential elements are excited by the secondary electrons, whereby plasma is generated. Also, the circumferential elements emit strong ultraviolet rays, and then the ultraviolet rays excite a fluorescent substance, thereby emitting visible rays.
In the edge-type method, a lamp unit is formed at one side of a light-guiding plate. The lamp unit includes a lamp, a lamp holder and a lamp reflecting plate. The lamp for emitting light is inserted into both sides of the lamp holder, whereby the lamp is protected from an external impact. Also, the lamp reflecting plate covers a circumferential surface of the lamp, and one side of the lamp reflecting plate is inserted to one side of the light-guiding plate to reflect the light emitted from the lamp to the light-guiding plate. Generally, the edge-type method for forming the lamp unit at the one side of the light-guiding plate is applied to relatively small sized LCD devices such as the monitors for the laptop type computer or the desktop type computer. The edge-type method is useful to obtain uniform luminance, long lifetime and thin profile in the LCD device.
With trend of the large-sized LCD device of 20-inch or more, the direct-type method is actively developed, in which a plurality of lamps are formed in one line on a lower surface of a light-diffusion plate, whereby the entire surface of the LCD panel is directly illuminated with the light. The direct-type method, which has greater light efficiency as compared with that of the edge-type method, is used for the large-sized LCD device requiring high luminance.
Hereinafter, a related art backlight assembly will be described as follows.
FIG. 1 is a schematic view for illustrating the related art backlight assembly.
As shown in FIG. 1, the related art backlight assembly includes a fluorescent lamp 1, a light-guiding plate 2, a light-diffusion substance 3, a reflecting plate 4, a light-diffusion plate 5 and a prism sheet 6. When a voltage is applied to the fluorescent lamp 1, electrons remaining in the fluorescent lamp 1 move to the anode, and the remaining electrons collide with argon Ar, whereby the argon Ar is excited. As a result, positive ions are generated, and the positive ions collide against the cathode, thereby generating secondary electrons. When the secondary electrons are discharged to the fluorescent lamp 1, the flow of the electrons collides with hydrargyrum vapor, and then ionized, thereby emitting ultraviolet rays and visible rays. Then, the emitted ultraviolet rays excite a fluorescent substance deposited inside the fluorescent lamp, thereby emitting light.
Subsequently, the light-guiding plate 2 is Wave-Guide to make the light emitted from the fluorescent lamp 1 be incident on the inside, and to emit a plate type light source. That is, the light-guiding plate 2 is formed of Poly Methyl Meth Acrylate (PMMA) having the great light transmittance. The light incidence of the light-guiding plate 2 is related with a ratio of the light-guiding plate thickness to the fluorescent lamp diameter, a distance between the light-guiding plate and the fluorescent lamp 1, and the shape of the reflecting plate. Generally, the fluorescent lamp 1 is slant on the center of the light-guiding plate 2 at the thickness direction, thereby improving the efficiency of light incidence. The light-guiding plate 2 for the backlight unit of the LCD device is divided into a printing-type light-guiding plate, a V-cut type light-guiding plate, and a scattering-type light-guiding plate.
Next, the light-diffusion substance 3 is comprised of SiO2 particles, PMMA and solvent. At this time, SiO2 particles having porosity are used for diffusing the light. Also, PMMA is used for adhering SiO2 particles to a lower surface of the light-guiding plate 2. The light-diffusion substance 3 is deposited on the lower surface of the light-guiding plate 2 in the dotted patterns, and the sizes of the dotted patterns are gradually increased to obtain a uniform plate-type light source on an upper surface of the light-guiding plate 2. That is, the dotted pattern has a small size in a unit area near to the fluorescent lamp 1, and the dotted pattern has a large size in a unit area apart from the fluorescent lamp 1. At this time, the shape of the dotted pattern may be varied. In case of the dotted patterns having the same size, the respective dotted patterns have the luminance of the same level regardless of the dotted shape.
The reflecting plate 4 is formed at the rear of the light-guiding plate 2, whereby the light emitted from the fluorescent lamp 1 is incident on the inside of the light-guiding plate 2. Also, the light-diffusion plate 5 is formed on the upper surface of the light-guiding plate 2, on which the dotted patterns are deposited, to obtain a uniform luminance at each viewing angle. The light-diffusion plate 5 is formed of PET or Poly Carbonate (PC) resin, and a particle-coating layer is formed on the light-diffusion plate 5 for diffusing the light.
Next, the prism sheet 6 is formed to improve the frontal luminance of the light transmitted and reflected to the upper side of the light-diffusion plate 5. That is, the prism sheet 6 transmits the light of the predetermined angle, and the light incident on the other angles is totally reflected, whereby the light is reflected to the lower side of the prism sheet 6 by the reflecting plate 4 formed on the lower side of the light-guiding plate 2. The backlight assembly having the aforementioned structure is fixed to a mold frame, and a display unit disposed at an upper side of the backlight assembly is protected by a top sash. Also, the backlight assembly and the display unit are received between the top sash and the mold frame being coupled to each other.
Hereinafter, a backlight unit of an LCD device according to the related art will be described with reference to the accompanying drawings. FIG. 2 is a perspective view for illustrating a backlight unit using a related art fluorescent lamp.
As shown in FIG. 2, the backlight unit includes a fluorescent lamp 11, a lamp housing 12, a light-guiding plate 13, a reflecting plate 14, a light-diffusion plate 15, a prism sneet 16, a protection sheet 17, and a main supporter 18. At this time, a fluorescent substance is coated on the inner surface of the fluorescent lamp 11 for emitting the light. Also, the lamp housing 12 fixes the fluorescent lamp 11, and concentrates the light emitted from the fluorescent lamp 11 on one direction. The light-guiding plate 13 provides the light emitted from the fluorescent lamp 11 to an upper side of an LCD panel, and the reflecting plate 14 is provided at the rear of the light-guiding plate 13 to guide the light leaking in an opposite side of the LCD panel toward the light-guiding plate 13. The light-diffusion plate 15 is formed above the light-guiding plate 13 to uniformly diffuse the light emitted from the light-guiding plate 13. Also, the prism sheet 16 is formed above the light-diffusion plate 15 to concentrate the light diffused in the light-diffusion plate 15, and to transmit the concentrated light to the LCD panel, and the protection sheet 17 is formed on an upper side of the prism sheet 16 to protect the prism sheet 16. The main supporter 18 receives and fixes the aforementioned elements.
In the aforementioned backlight unit, the light emitted from the fluorescent lamp 11 is concentrated on an incident surface of the light-guiding plate 13, and then the concentrated light passes through the light-guiding plate 13, the light-diffusion plate 15 and the prism sheet 16, whereby the light is transmitted to the LCD panel. However, the backlight unit of using the related art fluorescent lamp has a low color realization ratio due to the emission characteristics of a light source. Also, it is hard to obtain the backlight unit having high luminance due to limits in size and capacity of the fluorescent lamp.
Meanwhile, the backlight unit has been used for illuminating the screen of the LCD device, whereby the viewer can read information displayed on the screen in the dark surroundings. Recently, in the light-guiding plate of the backlight unit, it is required to obtain a thin light-guiding plate, a function for displaying various colors, and a formation of a Light Emitting Diode (LED) to satisfy demands for excellent design, low power consumption and thin profile.
Recently, many efforts have been made to obtain the thinness in the light-guiding plate for satisfying demands of excellent design and low power consumption. In addition, the LCD device has been developing to have a function for displaying various colors and a technical development for decreasing the power consumption with LEDs (light-emitting diode).
FIG. 3 is a plane view for illustrating a backlight unit of using an LED (Light Emitting Diode) according to the related art. As shown in FIG. 3, a plurality of red R, green G, and blue B LEDs 23a, 23b, and 23c are arranged at fixed intervals on a PCB substrate 21 of the rear surface of an LCD panel (not shown), whereby a light source 23 for emitting the light is provided. The LCD panel (not shown) is illuminated with the light emitted from the light source 23. Accordingly, the LCD panel displays the image in the dark surroundings.
The light source 23 is formed by arranging the red R LED 23a, the green G LED 23b and the blue B LED in one-dimensional structure on the PCB substrate 21.
To display the picture image on the LCD panel of the aforementioned backlight unit, a voltage is applied to the red R, green G and blue B LEDs 23a, 23b and 23c, whereby the red R, green G and blue B LEDs 23a, 23b and 23c emit the red, green and blue light. The red, green and blue light is mixed, so that the rear surface of the LCD panel is illuminated with the white light.
FIG. 4 is a plane view for explaining the method of emitting the white light by color mixing in the backlight unit having the LED according to the related art. As shown in FIG. 4, the monochromatic light of R, G and B emitted from the respective LEDs 23a, 23b and 23c is mixed to generate the white light. However, in the zone of ‘a’, there is the predetermined portion wherein the light emitted from the respective LED lamps is not overlapped, so that it is impossible to generate the white light. In the zone of ‘b’, the monochromatic light of R, G and B emitted from the respective LEDs is mixed, whereby the white light is generated.
The LED is used for the light source of the backlight unit in the LCD panel, so that it is possible to obtain the low power consumption and miniaturization in electronic equipments such as notebook PC, etc.
However, it is hard to mix the red, green and blue light emitted from the respective red, green and blue LED lamps, and to generate the white light by uniformly mixing the three colors, thereby lowering the light efficiency and color realization ratio. Accordingly, an LCD device and a method for driving the LCD are needed that substantially obviates the limitations of the prior art.